This invention relates generally to planing watercraft, and more particularly to such watercraft that employ hydrofoils for dynamic support thereof.
A primary concern of boat designers is the handling and performance characteristics of watercraft under a variety of adverse conditions including rough water, wave action against the hull, and drag produced from the flow of water past the hull. Optimum performance under these adverse conditions is a priority of the designer, and is traditionally accomplished by either hydrodynamic hull design, or less commonly by the incorporation of hydrofoils thereto.
Typically, most planing watercraft comprise a hydrodynamically designed hull that rides directly on the water. These watercraft depend entirely on water pressure against the hull for dynamic support at cruising speeds. Because these watercraft are designed to be in constant contact with the water, their performance is directly effected by rough water and drag.
More specifically, a planing boat supported entirely by water pressure against its hull, is exposed to wave forces that cause the boat to pitch up when waves are engaged. The pitching is caused because waves usually act on the forward section of the boat, somewhere between the bow and the center of gravity "C.G." Accordingly, the force of individual waves hitting the hull forward of the C.G. causes a bow up rotation, i.e. pitching movement. In this way, as a boat travels over the water, it repeatedly pitches in response to engaging waves. Such repeated pitching is the primary source of a rough ride.
In addition, when a planing boat travels on the water, a portion of the hull is usually in contact with the water. This portion of hull is referred to as the wetted area, and is the primary source of drag that a boat must over come. Because reduction of wetted area translates to reduction of drag, one objective of boat designers is to reduce wetted surface area of the boat's hull at cruising speeds.
One type of hull, designed to give pitch stability, and thus a smooth ride in rough water, is a deep-V shaped hull. Hulls having a deep-V shape, cut through waves with a minimum of pitching. However, because deep-V designs have a larger wetted surface, more drag is generated thereby requiring larger power plants and/or slower speeds. In addition, deep-V type hulls are somewhat less stable and tend to roll from side to side more than boats having flatter bottoms.
One way boat designers can minimize the effect of wave action and also reduce the wetted surface area of the hull is to incorporate hydrofoils in the boat's design. Boats that employ hydrofoils "foils" for dynamic support at cruising speed are well known in the art. Boat designers have, for many years, utilized a number of different arrangements of hydrofoils to reduce the effect of waves, and to reduce the power required to attain modestly high speeds. Generally, hydrofoils are classified in one of two groups: (1) surface piercing foils, and (2) fully submerged foils.
Surface piercing foils are the simplest in design because they are generally self-stabilizing in roll, and in height above the 'water. However, because a portion of the surface piercing foil is always in contact with the water surface, and therefore the waves, this type of foil is more susceptible to adverse effects of wave action that results in a rough ride.
In contrast, fully submerged foils have no contact with surface waves and therefore a smoother ride can be attained in rough water. Boat designers have found, however, that this type of design, where the hull raises out of the water and thus becomes airborne, is generally not passively stable, i.e., it is not self-stabilizing. Consequently, to maintain a specified height above water and a straight and level course, a boat having totally submerged foils usually requires an independent control system to adjust the angle of attack of the foil surface. This control is much like that of an aircraft requiring multiple control surfaces.
Because of these problems, the use of hydrofoils to improve watercraft performance has been somewhat limited. Several early designs incorporated hydrofoils to enhance the performance of watercraft having hulls designed to remain in contact with the water. For example, U.S. Pat. No. 3,964,417 to Williams et al. employs a forward planing hull along with two rear hydrofoils that provide additional lift. Because the hull of this design is somewhat flat and never leaves the water, the effect of waves and rough water on the front portion of the hull is substantial. Similarly, U.S. Pat. No. 3,881,438 to Jones incorporates front and rear hydrofoils to a hull designed to stay in contact with the water.
Subsequent similar designs include U.S. Pat. No. 4,665,853 to Gerdsen et al which discloses catamaran "side-by-side" hulls with front and rear hydrofoils that span between the hulls; U.S. Pat. No. 4,606,291 to Hoppe which also discloses a catamaran type hull with front and rear foils for enhanced dynamic support; and finally U.S. Pat. No. 4,915,048 to Stanford which discloses a front foil to generate a downward force, a rear foil to generate an upward force, and a rearwardly disposed step to cooperate with a stern pressure release zone.
In each of the above noted watercraft, wave impact on the hull occurs repeatedly and is well forward of the "center of gravity." Accordingly, an upward pitching takes place which results in a rough ride. To eliminate such pitching in watercraft, some designers turned to fully submerged hydrofoils incorporated on hulls that rise completely out of the water at cruising speed. As a result of the hull being completely separated from the water, wave action, and therefore pitching is minimized. However, with this type of design, a complex control system is usually required for longitudinal and lateral stability.
For example, U.S. Pat. No. 4,237,810 to West fall discloses a hydrofoil boat design that employs fully submerged foils for high efficiency longitudinal stability is maintained by a control mechanism that exerts pressure on the front strut to move an aerodynamic horizontal stabilizer which controls the pitch altitude.
Similarly, U.S. Pat. No. 4,962,718 to Gornstein discloses a boat with fully-submerged hydrofoils. Because of the location and distance of the foils from the hull, this boat design is not laterally or longitudinally stable in the absence of a stabilizing control system.
Finally, U.S. Patent to Cook discloses a watercraft having totally submerged foils attached to a hull that raises completely out of the water. Like Westfall and Gornstein, however, stability is maintained through a system requiring complex manipulation of foils.
As seen from the above, watercraft designs that incorporate hydrofoils for improved performance generally fall into one of two groups. Briefly, a first group employs a design where the hull maintains substantial contact with the water. Because of such contact, this design is susceptible to rough water conditions that cause pitching instability. A second general group comprise hulls designed to raise away from the water to reduce wave action from rough water, and to reduce wetted area drag. As noted, however, this type of design generally lacks passive stability when operating entirely foilborne. Additionally, such designs generally lack pitch stability when shifting from the foilborne mode to the waterborne mode.
Accordingly, a need remains for a safe, efficient, passively stable hydrofoil supported watercraft designed to operate foilborne or partially hullborne, while maintaining pitch stability when shifting from being foilborne to being hullborne or vice versa.